The number and variety of portable and mobile devices in use have exploded in the last decade. For example, the use of mobile phones, tablets, media players etc. has become ubiquitous. Such devices are generally powered by internal batteries, and the typical use scenario often requires recharging of batteries or direct wired powering of the device from an external power supply.
Most present day systems require a wiring and/or explicit electrical contacts to be powered from an external power supply. However, this tends to be impractical and requires the user to physically insert connectors or otherwise establish a physical electrical contact. It also tends to be inconvenient to the user by introducing lengths of wire. Typically, power requirements also differ significantly, and currently most devices are provided with their own dedicated power supply resulting in a typical user having a large number of different power supplies with each being dedicated to a specific device. Although, the use of internal batteries may avoid the need for a wired connection to a power supply during use, this only provides a partial solution as the batteries will need recharging (or replacing which is expensive). The use of batteries may also add substantially to the weight and potentially cost and size of the devices.
In order to provide a significantly improved user experience, it has been proposed to use a wireless power supply wherein power is inductively transferred from a transmitter coil in a power transmitter device to a receiver coil in the individual devices.
Power transmission via magnetic induction is a well-known concept, mostly applied in transformers, having a tight coupling between primary transmitter coil and a secondary receiver coil. By separating the primary transmitter coil and the secondary receiver coil between two devices, wireless power transfer between these becomes possible based on the principle of a loosely coupled transformer.
Such an arrangement allows a wireless power transfer to the device without requiring any wires or physical electrical connections to be made. Indeed, it may simply allow a device to be placed adjacent to or on top of the transmitter coil in order to be recharged or powered externally. For example, power transmitter devices may be arranged with a horizontal surface on which a device can simply be placed in order to be powered.
Furthermore, such wireless power transfer arrangements may advantageously be designed such that the power transmitter device can be used with a range of power receiver devices. In particular, a wireless power transfer standard known as the Qi standard has been defined and is currently being developed further. This standard allows power transmitter devices that meet the Qi standard to be used with power receiver devices that also meet the Qi standard without these having to be from the same manufacturer or having to be dedicated to each other. The Qi standard further includes some functionality for allowing the operation to be adapted to the specific power receiver device (e.g. dependent on the specific power drain).
The Qi standard is developed by the Wireless Power Consortium and more information can e.g. be found on their website: http://www.wirelesspowerconsortium.com/index.html, where in particular the defined Standards documents can be found.
The Qi wireless power standard describes that a power transmitter must be able to provide a guaranteed power to the power receiver. The specific power level needed depends on the design of the power receiver. In order to specify the guaranteed power, a set of test power receivers and load conditions are defined which describe the guaranteed power level for each of the conditions.
The Qi standard defines a variety of technical requirements, parameters and operating procedures that a compatible device must meet.
Communication
The Qi standard supports communication from the power receiver to the power transmitter thereby enabling the power receiver to provide information that may allow the power transmitter to adapt to the specific power receiver. In the current standard, a unidirectional communication link from the power receiver to the power transmitter has been defined and the approach is based on a philosophy of the power receiver being the controlling element. To prepare and control the power transfer between the power transmitter and the power receiver, the power receiver specifically communicates information to the power transmitter.
The unidirectional communication is achieved by the power receiver performing load modulation wherein a loading applied to the secondary receiver coil by the power receiver is varied to provide a modulation of the power signal. The resulting changes in the electrical characteristics (e.g. variations in the current draw) can be detected and decoded (demodulated) by the power transmitter.
Thus, at the physical layer, the communication channel from power receiver to the power transmitter uses the power signal as a data carrier. The power receiver modulates a load which is detected by a change in the amplitude and/or phase of the transmitter coil current or voltage. The data is formatted in bytes and packets.
More information can be found in chapter 6 of part 1 of the Qi wireless power specification (version 1.0).
Although Qi uses a unidirectional communication link, it has been proposed to introduce communication from the power transmitter to the power receiver.
System Control
In order to control the wireless power transfer system, the Qi standard specifies a number of phases or modes that the system may be in at different times of the operation. More details can be found in chapter 5 of part 1 of the Qi wireless power specification (version 1.0).
The system may be in the following phases:
Selection Phase
This phase is the typical phase when the system is not used, i.e. when there is no coupling between a power transmitter and a power receiver (i.e. no power receiver is positioned close to the power transmitter).
In the selection phase, the power transmitter may be in a stand-by mode but will sense in order to detect a possible presence of an object. Similarly, the receiver will wait for the presence of a power signal.
Ping Phase:
If the transmitter detects the possible presence of an object, e.g. due to a capacitance change, the system proceeds to the ping phase in which the power transmitter (at least intermittently) provides a power signal. This power signal is detected by the power receiver which proceeds to send an initial package to the power transmitter. Specifically, if a power receiver is present on the interface of the power transmitter, the power receiver communicates an initial signal strength packet to the power transmitter. The signal strength packet provides an indication of the degree of coupling between the power transmitter coil and the power receiver coil. The signal strength packet is detected by the power transmitter.
Identification & Configuration Phase:
The power transmitter and power receiver then proceeds to the identification and configuration phase wherein the power receiver communicates at least an identifier and a required power. The information is communicated in multiple data packets by load modulation. The power transmitter maintains a constant power signal during the identification and configuration phase in order to allow the load modulation to be detected. Specifically, the power transmitter provides a power signal with constant amplitude, frequency and phase for this purpose (except from the change caused by load-modulation).
In preparation of the actual power transfer, the power receiver can apply the received signal to power up its electronics but it keeps its output load disconnected. The power receiver communicates packets to the power transmitter. These packets include mandatory messages, such as the identification and configuration packet, or may include some defined optional messages, such as an extended identification packet or power hold-off packet.
The power transmitter proceeds to configure the power signal in accordance with the information received from the power receiver.
Power Transfer Phase:
The system then proceeds to the power transfer phase in which the power transmitter provides the required power signal and the power receiver connects the output load to supply it with the received power.
During this phase, the power receiver monitors the output load conditions, and specifically it measures the control error between the actual value and the desired value of a certain operating point. It communicates these control errors in control error messages to the power transmitter with a minimum rate of e.g. every 250 msec. This provides an indication of the continued presence of the power receiver to the power transmitter. In addition, the control error messages are used to implement a closed loop power control where the power transmitter adapts the power signal to minimize the reported error. Specifically, if the actual value of the operating point equals the desired value, the power receiver communicates a control error with a value of zero resulting in no change in the power signal. In case the power receiver communicates a control error different from zero, the power transmitter will adjust the power signal accordingly.
Qi originally defined a wireless power transfer for low power devices considered to be devices having a power drain of less than 5 W. Systems that fall within the scope of this standard use inductive coupling between two planar coils to transfer power from the power transmitter to the power receiver. The distance between the two coils is typically 5 mm.
However, work is ongoing to increase the available power, and in particular the standard is being extended to mid-power devices being devices having a power drain of more than 5 W. Other work is ongoing to extend the distance between the coils to e.g. 40 mm.
As mentioned, the Qi standard supports communication from the power receiver to the power transmitter, and this is used to enable the power receiver to communicate power requirements to the power transmitter which the power transmitter can then use to set characteristics of the generated power signal. Specifically, the power receiver may transmit power error control messages to the power transmitter and the power transmitter may control the transmitted power accordingly. Thus, a power control is implemented. The power transmitter is typically arranged to adjust the transmitted power by varying the duty cycle or the supply voltage to the drive circuit for the transmitter inductor, or by varying the frequency of the generated power transfer signal. As the transmitter inductor and the receiver inductor are typically part of a resonance or tank circuit, the change of frequency will also result in an overall change in the transferred power.
The communication from power receiver to power transmitter is achieved by the power receiver performing load modulation wherein a loading applied to the power receiver inductor is varied to provide a modulation of the power signal. The resulting changes in the electrical characteristics (e.g. variations in the amplitude of the current flowing through the primary coil) can be detected and demodulated by the power transmitter.
For load modulation, the power transfer signal generated from the transmitter inductor is accordingly used as a carrier signal for the load modulation introduced by the changes of the loading of the power transfer signal at the power receiver. In order to provide improved power transfer performance, it is of course necessary for the communication reliability to be as high as possible, and specifically for the bit or message error rate to be minimized. However, the load modulation performance depend on many different operating characteristics and parameters, including for example the frequency of the power transfer signal, the specific load values for different loads of the load modulation etc.
In order to optimize communication performance, it has been proposed in N. Y. Kim, S.-W. Yoon, and C.-W. Kim, “Frequency-agile load-modulated magnetic resonance wireless power transfer system for reliable near-field in-band signaling,” Electronic Letters, vol. 49, no. 24, pp. 1558-1559, Nov. 21, 2013, to implement a frequency tracking system which sets the carrier signal to a frequency that optimizes a modulation depth measured as the voltage amplitude difference for different loads of the power receiver when load modulating.
However, it has been found that such an approach tends to still lead to suboptimal performance, and to result in communication errors that adversely affect power transfer performance. Furthermore, the adjustment of the power transfer signal frequency based on communication considerations is typically impractical or not feasible for an efficient power transfer system where the frequency often needs to be optimized and/or is dynamically varied to provide the desired power transfer performance and characteristics.
An improved power transfer approach would accordingly be advantageous. In particular, an approach that allows improved operation, improved power transfer, increased flexibility, facilitated implementation, facilitated operation, improved communication, reduced communication errors, especially for load modulation, improved power control and/or improved performance would be advantageous. Especially, in many scenarios it would be advantageous to enable reliable communication at an increased distance between the power receiver and power transmitter coils since the reduced coupling is likely to result in lower demodulation depth.